The overall objective of the proposed research is to learn more about the primary site and mechanism of action of growth hormone (GH). Our studies with I125-labelled GH have shown that much hormone enters liver and kidney at short periods after administration, but the exact site of action is still unproven. One of the earliest changes observed in liver GH administration is a marked increase in ornithine decarboxylase (ODC) activity. Efforts to study this enzyme have been hindered by its low concentration in normal tissues such as liver. Our recent work has shown that much ODC activity is present in human placenta and purification studies presently are under way. These studies, which include a comparison of the properties of the placental enzyme with those of the enzyme from other sources, will be extended as the purification proceeds.